The present invention relates to a dental instrument for root canal therapy, and in particular to a dental instrument for piercing content filled in a root canal at the beginning of root canal therapy, and resecting if necessary a hard projection which appears on the root canal, to thereby make a route for allowing access of other dental instruments for root canal therapy.
In root canal therapy in dentistry, it is a common practice to resect a calcified root canal wall extending towards an apical portion and to remove resection debris and contents of the root canal to thereby expose a new surface of the root canal. In such therapy, dentists generally choose an instrument for root canal therapy most applicable to the target therapy from their suite of instruments categorized by sizes such as a reamer, K-file and H-file, and handle such instrument directly by a hand or indirectly as being attached to a hand piece, where advancement of the therapy is sensible through the operation of the instrument.
The reamer is composed of a triangle-sectioned or square-sectioned rod material twisted at a small angle, and mainly functions when being operated in a rotating manner so as to resect the root canal wall and remove resection debris and contents of the root canal. The K-file is composed of a triangle-sectioned or square-sectioned rod material twisted at a relatively large angle, and mainly functions when being operated in a reciprocal manner so as to resect the root canal wall and remove resection debris and contents of the root canal. The H-file is composed of a rod material shaped so as to have a sectional form surrounded by a part of straight line in the radial direction and a spiral curve, and mainly functions when being operated in a reciprocal manner so as to resect the root canal wall and remove resection debris and contents of the root canal.
The root canal has a curved and tapered form with the opening thereof shrinks toward the apical portion, where the curved form significantly differs from patient to patient. Thus the instruments for root canal therapy such as the reamer, K-file, H-file and so forth are designed to be highly flexible so as to fit themselves the root canal (root canal wall) which has a tapered and curved form differs with patients.
The instruments for root canal therapy such as the reamer, K-file, H-file and the like are preferably made of austenitic stainless steel which is not causative of fracture or chipping during the therapy and rusting. The applicant of the present invention holds a technology for fabricating various surgical needles and instruments for root canal therapy using austenitic stainless steel which is drawn in a fiber form by cold wire drawing so as to attain an excellent hardness and bending strength.
It is to be noted now that the root canal is filled with the content (dental pulp) including nerve, blood vessel, lymph vessel and so forth, which is sometimes found solidified. Thus the curved profile of the root canal or constricted state of the wall thereof due to calcification is not visible at the start of the therapy. When the therapy is started with an instrument for root canal therapy such as a reamer, K-file, H-file or the like, the tip portion of such instrument pierced into the content within the root canal is applied with a load which is ascribable to resistance during such piercing, and further applied with another load depending on the applied operational force when the tip portion penetrating the content comes into contact with the root canal wall.
Timing on which the tip portion of the instrument for root canal therapy comes into contact with the root canal wall varies depending on conditions such as curved profile of the root canal and the constricted state of the wall thereof due to calcification, so that it is difficult to regulate operation of the instrument for root canal therapy so as to reduce the force applied therethrough upon such contact, and instead the tip portion is applied with a large load upon such contact. The load often exceeds an allowable limit of such instrument for root canal therapy, and raises a problem that the instrument becomes unavailable due to a permanent deformation in the tip portion. Even known is a case in which the tip portion is folded, which is more serious problem surpassing wear of the instrument for root canal therapy.
Of course such therapy must be safety-oriented since the therapy is targeted at the root canal wall invisible due to the filled content. An instrument for root canal therapy larger in size and having a larger allowable load than that of a just-in-size instrument may solve the foregoing problem of the folded tip, but such instrument is more likely to be obstructed by the root canal wall and to fail in creating an access route. On the other hand, using an instrument for root canal therapy smaller in size may result in frequent deformation of the tip portion thereof, which consumes plural sizes of the instruments only in a single therapy.
The foregoing problem can be solved by beginning the therapy with creating an access route, which is helpful for guiding the instrument for root canal therapy, in the content filled within the root canal, and then by selecting a reamer, K-file, H-file or the like which is most suitable for such access route. To ensure such function to be exhibited, it is sufficient for the instrument for root canal therapy to have a strength durable enough against the load which is ascribable to resistance of the content in the root canal or to collision with the root canal wall.
In recent years, there is provided a therapeutic instrument used at the beginning of the therapy for creating an access route in the content filled in the root canal. The therapeutic instrument is made of a hardened carbon steel. Such therapeutic instrument is successful in obtaining larger bending strength as compared with that for an instrument made of austenitic stainless steel, so that it can effectively create in the root canal the access route for guiding other instruments for root canal therapy by reciprocal and rotating operation even when the root canal is filled with contents which is sometimes even solidified.
In the fabrication of such reamer and files, a round rod (wire) is directly ground to produce slant planes according to a desired taper so as to obtain a tapered and edged wire having a square, triangle or rectangular section. In the fabrication of the H-file, a round rod is directly ground with a single stroke.
As described in the above, a conventional problem resides in that the reamer, K-file and H-file are likely to be wasted during the root canal therapy after being excessively bended or permanently deformed due to load ascribable to the content in the root canal or contact with the root canal wall. This may result in use or wasting of excessive suite of the instruments for root canal therapy, and thus may further result in increased time and cost for the therapy. In some cases, such therapeutic instrument even cannot make the access route toward the apical portion.
On the other hand, hardening is indispensable for a therapeutic instrument which overcame the foregoing problem by using carbon steel. The portion to be hardened is, however, very thin and has only a small heat capacity, so that it is not easy to uniformly elevate the temperature of the entire portion of the instrument, and as a result to attain a uniform strength over the entire length of the instrument, which may also result in difficulty in ensuring uniformity of a number of the therapeutic instruments. This can even raise a serious problem that the suite of the instrument accidentally contain a brittle product which may chip within the root canal. Another problem resides in that the instrument made of carbon steel readily gets rust due to the specific nature of this material, and sterilizing such instrument once used in an autoclave may corrode and ruin the instrument. Removing the generated rust may degrade the strength below a desired level. Thus the instrument once used is to be disposed, which pushes up the cost for the instrument.
It is therefore an object of the present invention to provide a dental instrument for root canal therapy really capable of creating at the beginning of the therapy an access route for guiding other instruments for root canal therapy, and a method for fabricating such instrument at a low cost and with a high reliability.
The present inventors found out after extensive experiments for obtaining a dental instrument for root canal therapy free from the foregoing problems that the following factors are necessary to create in the content of a root canal an access route for guiding a reamer, K-file or H-file.
A first factor is that the tip portion of the instrument should have a bending strength enough to withstand the load generated when the tip portion is thrust into the content, and in particular for the case that the root canal is constricted due to curved profile thereof or calcification, that the tip portion of the instrument should have a bending strength enough to withstand the load generated when the tip portion accidentally comes into contact with the root canal wall at the constricted portion.
Since the instrument can achieve an object only by such thrusting, it is necessary, while considering a bending strength of a similarly operated K-file as a standard, for the instrument of the present invention to have a strength sufficiently larger than that of the K-file.
Selecting a material with a high strength is important for improving the bending strength. A material essentially requires hardening, however, tends to cause hardening-related dispersion; and a material likely to rust does not allow preliminarily sterilization, has difficulty in the repetitive use, and has only a limited duration of the storage. It is thus preferable to use austenitic stainless steel which is not causative of rusting, and it is also preferable that the material is drawn in a fiber form by cold wire drawing so as to attain an excellent hardness and bending strength.
Using a cold-drawn austenitic stainless steel and defining a certain sectional form of the instrument will desirably improve the bending strength.
A second factor is that the content filled in the root canal can readily maigrate. It is not always necessary to remove the content outside the root canal, and only dislocation within the root canal so as to allow creation of the access route can suffice. To dislocate the content within the root canal, it is essential to ensure a certain gap, which allows such content to migrate, between the tip portion of the dental instrument for root canal therapy and the root canal wall.
A third factor is that the resistance caused by the piercing of the instrument into the content of the root canal should be small as possible. It is necessary for the instrument used herein that the surface of the portion pierced into the content should have a less amount of irregularity. In particular when the instrument is pierced into the content, a subtle touch upon contact of the tip portion with the root canal wall or other materials should be sensible by a dentist operating such instrument. Thus while being preferably low, the resistance is also required to have a certain level so that a dentist can sense it.
A fourth factor is that the instrument should have a minimum cutting edge so as to cut a constricted portion of the root canal due to the curved profile or calcification. The instrument, originally designed to create an access route, should be extra thin, where the tip portion of which preferably has a diameter of 0.06 mm to 0.15 mm, and more preferably 0.06 mm to 0.10 mm. Thus the instrument is flexible enough to bend along the curved root canal. It is thus necessary for the instrument to have a rigid cutting edge capable of cutting the solidified content or projections thrust out from the root canal wall.
A dental instrument for root canal therapy according to the present invention which can satisfy the above factors has a shaft portion and an operational portion extended from such shaft portion, wherein at least a region from the tip of the operational portion down to a predetermined distant position has a sectional form surrounded by an arc and a chord, and a length of a line segment along a vertical bisector of the chord sectioned between such chord and such arc equals to or longer than five-eighths of the diameter of a virtual circle composed by such arc.
Using such dental instrument for root canal therapy, an effective access route can be created in the content filled in the root canal at the beginning of root canal therapy. (The dental instrument for root canal therapy of the present invention is hereinafter referred to as xe2x80x9cpiercing instrumentxe2x80x9d since the instrument is intrinsically targeted at creating an access route).
Since the piercing instrument has a region from the tip of the operational portion down to a predetermined distant position whose sectional form is surrounded by an arc and a chord, and a length of a line segment along a vertical bisector of the chord sectioned between such chord and such arc equals to or longer than five-eighths of the diameter of a virtual circle composed by such arc, the instrument can have a section whose area and sectional secondary moment are larger than those of K-file, a conventional instrument for root canal therapy pierced into the content of the root canal in the first stage of the therapy. Thus the instrument can obtain bending strength and bending rigidity significantly higher than those of the K-file.
Composing the sectional form with an arc and chord can facilitate the migration of the content within the root canal.
In particular, the edges formed at the joint portions of the chord and arc can function as a cutting edge. Thus for the case that a part of the calcified root canal wall is present in a form of projection, rotating such piercing instrument can successfully resect the projection. Despite a round section is generally preferred for the piercing instrument in terms of bending strength, the present invention employs a sectional form surrounded by a chord and an arc considering the function of such cutting edge.
Another piercing instrument according to the present invention has a shaft portion and an operational portion extended from such shaft portion, wherein at least a region from the tip of the operational portion down to a predetermined distant position has a sectional form surrounded by an arc and two or three chords, a cross section thereof forming a D-shape, a length of the shortest line segment along a vertical bisector of the chord sectioned between such chord and such arc or between two chords equals to or longer than five-eighths of the diameter of a circle composed by such arc, and a length of the arc equals to or longer than a half of the circumference of a virtual circle composed by such arc.
According to such piercing instrument, cutting ability can slightly be increased as compared with the above-described piercing instrument without ruining the bending strength, while retaining a good operability in the piercing by virtue of the arc having a length equals to or longer than a half of the circumference of a virtual circle composed by such arc.
The present invention also provides a method for fabricating a dental instrument for root canal therapy having a shaft portion and an operational portion extended from such shaft portion, and at least a region from the tip of the operational portion down to a predetermined distant position has a sectional form surrounded by an arc and a chord, comprising a step for producing a tapered cylinder within a region at least from the tip of the operational portion down to a predetermined distant position; and a step for removing the sidewall portion of the cylinder so as to produce a predetermined plane.
Fabricating the piercing instrument by such method can successfully provide the product with an excellent quality at a low cost.